signature SET =
sig
  type 'a set

  val emptyset   : 'a set
  val isin       : ''a -> ''a set -> bool
  val addin      : ''a -> ''a set -> ''a set
  val removefrom : ''a -> ''a set -> ''a set 
end;

structure Set =
struct
  type 'a set = 'a list;

  val emptyset = [];

  val rec isin =
	fn x =>
		(fn []   => false
		  | y::l =>
			if (x = y)
			    then
				true
		  	    else
				isin x l);

  val addin =
	fn x =>
		 fn l =>
			if (isin x l)
			    then
				l
			    else
				x::l;

  val rec removefrom =
	fn x =>
		(fn [] => []
		  | y::l =>
			if (x = y)
			    then
				l
		  	    else
				y::(removefrom x l));
end :> SET;